Deposits of caesium 137 coming from old atomic tests and remains of the Chernobyl accident are responsible for both external and internal irradiations. Caesium 137, which emits gamma rays and beta electrons, contaminates layers of soil and plants. The campers on the left of the image are directly exposed to the gamma rays emerging from the ground - gamma rays which are distinctive of caesium and thus make for easy identification. As shown on the right, wild boars and mushrooms accumulate stockpiles of caesium, which is thus concentrated into the food chain. It is beta electrons which then become the primary source of internal irradiation. It must be remembered, however, that several months of camping and many portions of wild boar are needed for doses of even 1 millisievert to be absorbed.